Field of the Invention
The present invention relates to a hearing instrument comprising an audio signal source (typically a microphone arrangement) for providing audio signals, an audio signal processing unit for processing the audio signals, means to be worn at or in a user's ear or to be implanted in the user's head for stimulating the user's hearing according to the processed audio signals, and a rechargeable power source.
Description of Related Art
Hearing instruments are typically powered by an electrochemical battery which may be replaceable (disposable) or rechargeable. Rechargeable batteries store electric energy in chemical form. Since the maximum charging current typically is relatively low, charging of batteries takes quite a long time. Further, the lifetime of rechargeable batteries is limited by the number of charging cycles. However, the energy density is relatively high.
Another option for storing electric energy is to use capacitors, wherein the electric energy is stored as an electric field between two capacitor plates. Wearing of the capacitor due to the charging cycles is very small, and the charging currents may be high. However, the energy density typically is much lower than in electrochemical batteries.
A specific type of capacitors having high energy densities is an electrochemical double layer capacitor (EDLC), wherein an electrolyte layer between the capacitor plates is formed and wherein an electrically conducting material having very high surface area, such as activated carbon, is wetted by the electrolyte. The contact of the electrolyte with the surface of the large surface material creates an electrical double layer of charge carriers of opposed polarity having a thickness of only a few molecular layers which do not move. A separator layer is provided between the electrodes in order to prevent direct electric contact between the two electrodes. Such EDLCs have a very high energy density.
U.S. Pat. No. 8,224,007 B2 relates to a rechargeable electrochemical battery to be used in a hearing aid, wherein the battery is based on thin film technology and consists of a wire-like structure which is wound to form an induction coil which may be used both for charging purposes and as a transmitting or receiving antenna for communication purposes. The wire-like structure consists of a radial sequence of concentric hollow cylindrical layers forming the current collectors, anode, cathode and electrolyte.
International Patent Application Publication WO 99/55131 A2 relates to an electrical energy storage unit for a hearing aid, wherein a foil-like capacitor, i.e., a capacitor film, is provided in the shape of a hollow cylinder which is surrounded by an induction coil used for charging of the capacitor coil.
German Patent Application DE 36 23 304 A1 relates to a film capacitor which is wound in the shape of a hollow cylinder forming an induction coil, with such capacitor arrangement being used for creating high energy electromagnetic pulses. One end of one of the two electrode foils and the opposite end of the other one of the electrode foils are provided with a contact, with the electric circuit for discharging of the capacitor being closed by closing a switch provided between the two contacts, whereby the discharging current creates an electromagnetic induction field. Also examples of a capacitor having more than two electrode layers are described.
European Patent Application EP 0 376 948 B1 relates to an ignition system of internal combustion engines comprising a capacitor comprising two metallic conductor sheets wound in a role, wherein the wound capacitor also serves as the inductance required in the ignition circuit to generate ignition sparks.
U.S. Pat. No. 4,048,593 relates to an inductive-capacitive network used as a filter or a transformer, The network comprises a structure consisting of one or more sheets of dielectric material having one or more conductive sheets positioned on each dielectric sheet, wherein the sheet assembly is turned into a spiral configuration.
U.S. Pat. No. 7,023,687 B2 relates to a capacitor having a very high energy density which is made of a porous shaped body of a catalyst support material, wherein the surface of the porous body is coated with a first electrically conductive layer, a barium-titanate layer and a second electrically conductive layer.